PERS p ECTIVES Drosophila mutants was awarded the Nobel TIM e LIN e Prize in Physiology or Medicine in 1995 (TIMELINE). Today, the term Wnt is therefore Wnt signalling and its impact on an amalgam of Wg and Int10. There had been some earlier work on Wnt signalling, in ‘precloning’ times, when development and cancer the underlying pathways and mechanisms had not been identified. In the 1930s, viral Alexandra Klaus and Walter Birchmeier insertion was discovered to promote mam- mary tumours in laboratory mice (see Ref. 11 Abstract | The Wnt signalling pathway is an ancient system that has been highly for an example). Even earlier, in a famous conserved during evolution. It has a crucial role in the embryonic development experiment conducted in 1924, Mangold and of all animal species, in the regeneration of tissues in adult organisms and in Spemann grafted dorsal lips of the blastopore many other processes. Mutations or deregulated expression of components of from developing newt embryos onto the the Wnt pathway can induce disease, most importantly cancer. The first gene to opposing side of the embryo, inducing a 12 be identified that encodes a Wnt signalling component, Int1 (integration 1), was second body axis — a twin-headed embryo . The cause was the activity of the protein later molecularly characterized from mouse tumour cells 25 years ago. In parallel, the termed Wnt in the transplanted tissue frag- homologous gene Wingless in Drosophila melanogaster, which produces ment13. This work was awarded the Nobel developmental defects in embryos, was characterized. Since then, further Prize in 1935. Moreover, in an experiment components of the Wnt pathway have been identified and their epistatic performed in 1902 by Morgan, the simple salt relationships have been defined. This article is a Timeline of crucial discoveries lithium chloride also induced double axes in frog embryos by activating the pathway later about the components and functions of this essential pathway. termed the Wnt signalling pathway14,15. We know today that Wnt and a handful of developmental genetics, cell biology, cancer The canonical Wnt signalling pathway other signalling systems (Notch, Hedgehog, research, biochemistry and immunology. Following the discovery of Int1, for almost TGFβ (transforming growth factor-β)–BMP In addition, Wnt research has covered the 10 years most successful research into the (bone morphogenetic protein) and recep- spectrum of model organisms, including Wnt pathway was in the developmental tor tyrosine kinases) are major molecular worms, flies, frogs, mice and humans field, before the link to human cancer was mechanisms that control embryonic (Supplementary information S1 (table)), and realized (see next section). Many of the development. These signalling systems oper- therefore serves as an example of successful genes in the Wnt pathway, which were ate beyond cell and tissue boundaries, but interdisciplinary research. first discovered to function transiently function as morphogens that are secreted in development, turned out to act as from one cell or tissue type to activate surface Early discoveries oncogenes and tumour suppressors when receptors, signal transduction components In 1982, Roel Nusse and Harold Varmus, deregulated in human cancer. Thus, several and transcription factors in neighbouring then working at the University of of the Drosophila mutants identified by cells or tissues, regulating processes such as California, San Francisco, USA, reported Nüsslein-Volhard and Wieschaus’s genetic cell proliferation, survival or differentiation. that a tumour virus (mouse mammary screen in the late 1970s and early 1980s8,9, During development, the activity of such sig- tumour virus, MMTV) induced mammary and in other screens (for example, that of nalling systems is tightly regulated, whereas gland tumours in mice by activating the Perrimon and collaborators16,17), displayed in cancer and other diseases they can escape expression of a hitherto unknown gene that defects in embryonic segmentation — that this control. For example, a signalling they named Int1 (integration 1)1 (TIMELINE). is, segment polarity defects — similar to component that functions transiently during A spontaneous loss-of-function mutation Wingless mutants. These were caused by development might become an oncogene in the mouse, swaying, that lacked the ante- mutations in armadillo (β-catenin in ver- when it undergoes a gain-of-function muta- rior cerebrellum (and was first described tebrates), dishevelled or porcupine genes, as tion. Alternatively, an inhibitor might suffer in 1967 (REF. 2)) was shown to be a mutant shown by the groups of Wieschaus, Nusse, a loss-of-function mutation, lose its ability to allele of Int1 (RefS 3,4). A Drosophila mela- Perrimon and collaborators8,9,17–22. Whereas regulate signalling and lose its functions as a nogaster mutant lacking wings, Wingless wild-type embryos contained segments tumour suppressor. Both types of change in (Wg), was described in 1973 (Ref. 5), and with alternating rows of spikes and naked Wnt signalling have been linked to cancer. this fly gene turned out to be the homo- belts, segments in mutants contained only Therefore, a great deal of effort is being logue of mammalian Int1 (RefS 6,7). The spikes. By contrast, mutations such as zeste invested worldwide in developing therapeutic Wg mutation also caused segmentation white 3 (also known as shaggy; encodes agents that function by fine-tuning the Wnt defects in Drosophila embryos, and a glycogen synthase kinase 3β) caused oppo- pathway. number of segment polarity gene mutations site phenotypes; that is, completely naked This article describes major milestones in Drosophila had already been produced segments. Epistatic analysis of double that have substantially contributed to and extensively characterized by Nüsslein- mutants in the early 1990s demonstrated our understanding of the Wnt signalling Volhard and Wieschaus8,9. Subsequently, that these segment polarity genes function system. The history of Wnt research reads the developmental phenotypes were traced as components of a newly discovered signal like a survey of disciplines and benchmarks to mutations in components of the Wnt transduction pathway, the canonical Wnt in modern research, drawing together signalling system and the work on these pathway19–25 (TIMELINE). nature reviews | CANCER vOLume 8 | maY 2008 | 387 © 2008 Nature Publishing Group PERS p ECTIVES Timeline | Milestones in the 25 years since the discovery of the first Wnt gene Wingless and Shaggy Nuclear accumulation of β-catenin was found in were found to trigger colorectal cancers135. opposing changes in stability and Porcupine, a multi-transmembrane protein, was phosphorylation of found to process Wnt ligands54. Armadillo82,83. Int1 was found to be Mutations in the gene β-catenin asymmetry and nuclear location was identical to Wingless in adenomatous polyposis Dishevelled was found to regulate embryonic axis formation in Drosophila coli (APC) were found in identified as an essential Xenopus225,226. melanogaster6,7. patients with FAP and in element in the Wnt sporadic colorectal β-catenin was pathway21,24. Frizzled (FZD), a seven-span-transmembrane Inherited human cancers119,120,223. molecularly cloned224. receptor, was identified as the cell-surface colorectal cancer, familial receptor of Wnt ligands41,42. adenomatous polyposis Injection of Wnt1 mRNA LEF (lymphoid enhancer (FAP), was associated with caused the formation of a factor)–TCF (T-cell factor) APC was found to β-catenin was found to directly interact with LEF– deletions of chromosome secondary body axis in transcription factors were directly interact with TCF transcription factors, to be translocated to 5q21–5q22117,118. Xenopus laevis embryos26. molecularly cloned33–35. β-catenin113,114. the nucleus and to modulate gene expression30-32. 1982 1987 1989 1990 1991 1992 1993 1994 1995 1996 1997 Roel Nusse and New Wnt pathway A dominant mutation The mouse Min APC was found to regulate β-catenin Stabilizing β-catenin mutations Harold Varmus found components were (Min) in mice predisposes mutation was found stability81. were identified in melanoma and that Int1, a mouse found in a screen of to multiple intestinal to be a truncation colon cancer cell lines128–130. protooncogene, was lethal mutations in neoplasia126. mutation in Apc127. The Nobel Prize in Physiology or associated with Drosophila17. Medicine was awarded to Nüsslein- The three-dimensional structure MMTV-induced Accumulation of Shaggy was shown to Volhard, Wieschaus and Lewis for their of β-catenin was determined227. mammary gland Armadillo in Drosophila be a cytoplasmic discoveries of the genetic control of early tumours1. was found to be mediator of Wnt embryonic development in Drosophila. Phosphorylation targets β-catenin regulated by Wingless19. signalling22. to ubiquitylation, involving interaction with the E3 ligase β-TrCP, and to proteasome- dependent degradation89–92. Axin1 (the classical murine gene Fused) encodes Soluble Frizzleds (SFRP2 and FRZB) were identified as Identification of the homeotic gene an intracellular inhibitor of Wnt signalling36. soluble extracellular antagonists of Wnt ligands71–74. Ubx as the first Wnt target gene107. Blue boxes indicate discoveries of Wnt pathway components in cancer Axis duplication in frog embryos (the TOPflash reporter assay, was developed by the basic mechanism and the components of experimental model system of Xenopus lae- Clevers and collaborators and is now used